Threshers, the chain of off-licenses that was swarmed by customers last year after publishing online vouchers offering huge savings on champagne and wine, launched an almost identical scheme yesterday for this year's Christmas season.

Many branches of Threshers came close to running out of stock last year, after what seemed to have been intended as a limited internet promotion snowballed. The company had sent out vouchers to key suppliers and to staff members; the discounts were ultimately emailed to shoppers all over the UK.

Threshers honoured the vouchers, while insisting that they had been aimed only at a specific few and warning that the company could suffer serious financial consequences by being forced to sell large volumes of stock at such low prices.

However, marketing experts expressed suspicions that the company had actually pioneered a very clever sales tool, taking advantage of viral marketing techniques in order to provide a huge boost to pre-Christmas sales.

Yesterday's launch of a very similar scheme for this year will compound those suspicions. "Following its great success in 2006, we have once again decided to introduce our 40-per-cent-off-wine-and-champagne voucher for friends and family in the run-up to the Christmas period," said a company spokesman. "This special offer will run from 29 November until 9 December and vouchers will be redeemable in-store."

Within hours of release, the Threshers vouchers had once again been widely emailed to UK consumers.

The huge interest in last year's initiative has encouraged other retailers to engage in similar promotions, with some controversy. Littlewoods, for example, became embroiled in a row with customers this year after claiming that some had not been entitled to use their discount vouchers. Last week, Gap was forced to warn customers about fake vouchers issued in its name.

However, promotions of this type could be a key weapon for retailers desperate to record strong sales over the Christmas period in the face of mounting concerns about a slowdown in consumer spending.

A CBI survey of retail sales warned yesterday that while the latest figures from the high street were not as gloomy as some had predicted, the outlook for the Christmas period remained uncertain a concern for many retailers, given that the festive season accounts for as much as 60 per cent of their annual turnover. The CBI said: "Retailers will need a last-minute rush if hopes for a strong performance are to materialise."

Malcolm Pinkerton, an analyst at the British Retail Consortium, said that "margins are definitely being squeezed" at retailers. However, he said that last year's use of viral email marketing had "proved successful for many".

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